The present invention relates to strain energy accumulators. A strain energy accumulator may be provided as part of a hydraulic energy storage system with a reversible pump/motor in a vehicle hybrid hydraulic drive system. The hybrid hydraulic drive system absorbs and stores drive energy in the form of a working fluid by pumping the working fluid from a low pressure reservoir into the expandable strain energy accumulator. The hybrid hydraulic drive system provides drive power to the vehicle by using the stored high pressure fluid from the accumulator to operate the pump/motor as a motor. Hybrid hydraulic drive systems can thus add power to or subtract power from a conventional vehicle drive system of the vehicle.